1. Field of the Invention
The present invention relates to the field of transmission mechanisms for use in automatic gearboxes, and more particularly relates to such a transmission mechanism which provides various speed stages by use of two selectively engagable power transmission systems each of which is equipped with a clutching mechanism.
2. Description of the Prior Art
There is a known type of transmission mechanism for providing various speed stages between a first rotating member and a second rotating member, which comprises two power transmission systems, each of which can selectively be engaged to provide transmission of rotational power at its own particular speed ratio (or typically any selectable one of a plurality of ratios) between the first and second members, and each of which has a clutching mechanism which either can be connected so as to thus transmit rotational power or can be disconnected so as not thus to transmit rotational power. By appropriately controlling the engagement and disengagement of the clutching mechanisms, and concurrently controlling the selection of the various speed ratios of the various power transmission systems, which may be done automatically by a transmission control system of a per se well known type so as to provide entirely automatic operation, it is therefore possible to automatically shift between a plurality of speed stages even when the engine is under load and to provide both powered and engine braking operation of the transmission as a whole, and also to provide a starting off from rest functional capability, without the use of any fluid torque converter, which is a very valuable saving in the construction of an automatic transmission for vehicle use.
However, this prior art type of transmission mechanism has suffered from the disadvantage the such a mode of operation requires close and accurate control of the timing of the engagement and disengagement of the various clutching mechanisms. If the engagement of one of the clutching mechanisms occurs somewhat too late after the disengagement of the previously engaged transmission mechanism, then for a certain intermediate period the engine associated with the transmission will race, especially during a power on speed stage shift when the engine is under load as during upshifting of the transmission during acceleration or during a kick down type downshift, and this can damage various elements of the transmission mechanism; and on the other hand if the engagement of one of the clutching mechanisms occurs somewhat too early before the disengagement of the previously engaged transmission mechanism is complete, then for a short time period very large torsional strains are liable to be put on various elements of the transmission, in consequence of such an attempt, effectively, to engage two different speed ratios of the transmission at one time, and this also can damage various elements of the transmission mechanism. Such a thusly necessitated close and indeed subtle control of the timing of the engagement and disengagement of the various clutching mechanisms has presented a severe disadvantage with respect to the realization of an automatic transmission incorporating such a transmission mechanism, because such close timing control is very difficult to ensure, especially after the transmission has been used for a long period of time and its operational time constants have altered with usage. For example, in the case of a hydraulic type transmission control system, changes in the viscosity of the hydraulic fluid and changes in the size of the various orifices of the control system can cause timing variations of a magnitude sufficient to invalidate any attempt to perform such subtle control. Especially, good timing control is important in the case of power applied on downshifting of the transmission or a so called kick down downshift, when engine load is high, and snappy and accurate shifting of the speed stages of the transmission mechanism is particularly important and also difficult to provide in such a case.
Another factor that is required to be taken account of in the design of a transmission mechanism for an automatic transmission is mountability in a vehicle. The friction engaging elements of a transmission such as the hydraulic clutches thereof are liable to be quite large in diameter, especially as compared with the diameters of the various gear wheels of the transmission, and especially in the case of a front engine rear wheel drive type of vehicle such a transmission mechanism as outlined above has presented the constructional difficulty that the amount of space available for mounting such large diameter clutching mechanisms has not been very generous, since typically the rear end of an automatic transmission which utilizes such a transmission mechanism has been required to be housed at least partly under the floor of the passenger compartment of the vehicle, perhaps within a so called transmission tunnel or transmission hump thereof. On the other hand, in the case of a front engine, front wheel drive type vehicle such a transmission mechanism as outlined above has presented the constructional difficulty that the amount of axial space available for mounting the transmission has been very limited, which has limited the axial length of the transmission as a whole.
Finally, it is almost a required characteristic of such a transmission mechanism that it should provide engine braking for the vehicle, at least in some of the speed stages thereof. Smooth starting off from rest of the vehicle, both forwards and backwards, are also very important, and it should not be necessary for unduly delicate control of the clutching mechanisms to be exerted in order to obtain this effect. Power loss in the transmission should be minimized. Further, within these above mentioned constraints, it is a general design objective of such an automatic transmission mechanism to be as light and compact as possible, and to have as short an axial length as possible.